<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      background-color: #f00; /* #FF0000 => rgb(255, 0, 0) */
      color: white;

      /* text-align */
      text-align: center;
    }
  </style>
</head>
<body>
  
  <div class="box">我是div元素</div>

</body>
</html>